1. Field of the Invention
The present invention relates to an active matrix organic EL (electroluminescence) display and its gradation control method. In particular, the present invention relates to a driving circuit, a driving method and a gradation control driving method for an active matrix organic EL display.
2. Description of the Related Art
It can be said that, as compared with another type of display, an active matrix organic EL display is excellent in a wide field angle, high speed of response, a thin and light body, and the like.
Incidentally, “Organic Electroluminescent Diodes”, C. W. Tang and S. A. Vanslyke, Appl. Phys. Lett. 51, p. 913, 1987 describes the basic device structure of an organic EL device which constitutes the active matrix organic EL display. FIG. 13 indicates the structure of the conventional organic EL device. In FIG. 13, an ITO (indium tin oxide) electrode 202 being a transparent electrode, aromatic diamine 203 being an HTL (hole transport layer), a tris-aluminum complex (Alq3) 204 being an organic emitter layer, and magnesium-aluminum alloy (Mg:Ag) 205 being a cathode are sequentially laminated on a glass substrate 201. Then, light is picked up from the side of the ITO electrode 202 which is transparent in regard to visible light. By the structure of the organic EL device illustrated in FIG. 13, it is possible to emit light at external quantum efficiency 1% or more and luminance 1000 cd/m2 by the driving voltage of 10V or less.
Next, driving systems for causing the organic EL device to emit light will be described.
More specifically, there are two kinds of driving systems for causing the organic EL device to emit light. That is, the driving systems include a passive matrix system and an active matrix system. The passive matrix system is characterized in that the constitution is simple and its manufacturing cost can be made low. In the passive matrix system, a selection line is selected one by one to perform light emission for a pixel. Since the display time of one pixel is constant, the number of the selection lines is in inverse proportion to the light emission time for each selection line. For this reason, in a high-precision device, since the light emission time must be shortened, it is necessary to instantaneously flow a large current to each pixel. This is the serious factor for shortening a lifetime of the organic EL device.
FIG. 14 indicates light emission luminance during one frame period in the passive matrix system. That is, in the one frame period, the period that light emission of the pixel is being performed is only a selection period. As illustrated in FIG. 14, the driving system in which no light emission state occurs until a next video signal is input can be considered as one example of “impulse type driving”.
On the other hand, FIG. 15 indicates a basic circuit in the active matrix system. As illustrated in FIG. 15, two kinds of transistors, a switching TFT (thin film transistor) 404 and a driving TFT 405, are provided for each pixel. In the one frame period, during the selection period that a selection line 401 is “high”, the switching TFT 404 is “on”, and a predetermined voltage is applied to a data line 402, whereby the relative voltage is programmed (set) to a storage capacitor 407. Further, in the one frame period, during the non-selection period that the selection line 401 is “low”, the driving TFT 405 is driven according tot he programmed voltage, whereby a current flows from a voltage supply line 403 to an organic EL device 406.
In the above active matrix system, since light emission can be continuously performed even in the non-selection period, maximum luminance for each pixel can be suppressed, whereby reliability increases.
FIG. 16 indicates the light emission luminance during the one frame period in the active matrix system. In the one frame period, light emission of the pixel continuously performed at the light emission luminance programmed in the selection period. As illustrated in FIG. 16, the driving system in which the light emission state is maintained until a next video signal is input is called “hold driving system”. In the hold driving system, in addition to a voltage program (voltage setting system) for designating the light emission luminance based on a voltage as illustrated in FIG. 15, a current program (current setting system) for designating the light emission luminance based on a current value is known.
In each of the impulse-type driving and the hold-type driving described as above, the following gradation display is performed.
FIGS. 17A to 17D schematically indicate a gradation display method according to the impulse-type driving. More specifically, FIGS. 17A to 17D indicate a case of designating four gradations, for example, gradation 0 to gradation 3. In such an example, an amplitude value of the voltage or the current to be applied to the organic EL device in each selection period is modulated for each gradation, thereby performing gradation control. Here, to set the more number of gradations, it only has to minutely set the amplitude value of the voltage or the current for each gradation.
Incidentally, Japanese Patent Application Laid-Open No. 2000-056727 discloses a driving apparatus for achieving high gradation by properly combining pulse width modulation and amplitude value modulation.
FIG. 18 indicates an output of the current or the voltage value of the driving apparatus, described in Japanese Patent Application Laid-Open No. 2000-056727, in which the pulse width modulation and the amplitude value modulation are properly combined. In such an example, during an effective scanning period of one horizontal period, that is, during a selection period, a pulse width of a current or voltage value is expressed by four bits and 16 gradation and an amplitude value thereof is expressed by four bits and 16 gradations. Namely, the pulse width and the amplitude value of the current or voltage value are totally expressed by eight bits and 256 gradations. Since four-bit coding in a time direction is performed for 0, 1, 1, 2, 4, 8, instead of usual 0, 1, 2, 4, 8. This is because, since the width of usual coding starts from 0, one LSB unit is added in the time direction.
On the other hand, the gradation display is performed in the hold-type driving as follows.
FIGS. 19A to 19D schematically indicate a gradation display method according to the hold-type driving. More specifically, FIGS. 19A to 19D indicate a case of designating four gradations, for example, gradation 0 to gradation 3. In such an example, the voltage to be applied to the storage capacitor 407 in each selection period is modulated for each gradation to hold a certain voltage at the storage capacitor 407, thereby performing gradation control as maintaining the light emission state even in the non-selection period. Here, to set the more number of gradations in the hold-type driving, it only has to minutely set the voltage of the storage capacitor for each gradation.
In the above examples, light emission is performed only in the selection period in the impulse-type driving, luminance decreases if the number of selection lines is increased for achieving highly precise operation. Further, since it is necessary to instantaneously flow a large current to each pixel in the selection period so as to improve luminance, a lifetime of the organic EL device is shortened. Furthermore, since the selection period shortens if the number of selection lines increases, it becomes difficult to perform pulse width modulation as described in Japanese Patent Application Laid-Open No. 2000-056727.
For example, in a case where the number of selection lines is 1080 and the frame rate is 120 frames/second, if the maximum luminance is set to 500 cd/m2, the maximum light emission luminance of 540000 cd/m2 is necessary for the selection period of each pixel. Further, if the number of selection lines is 1080 and the frame rate is 120 frames/second, the selection period is 7.7 μsec at the maximum. Thus, if the three-bit division is performed as illustrated in FIGS. 7A to 7C, the minimum pulse width comes to be 1 μsec or less. In addition, in regard to the intermediate gradation, there is a case where the current value to which the maximum luminance of 540000 cd/m2 is necessary with the pulse width of 1 μsec or less is output. Accordingly, high output and high-speed operation which are extremely hard to the driver are required.
On the other hand, in regard to the hold-type driving, such a problem of high-speed operation as in the impulse-type driving does not easily occur since the light emission state is maintained even in the non-selection period. However, another problem occurs if the number of gradations increases in the hold-type driving. That is, unlike the impulse-type driving, since the maximum current value or the maximum voltage value is relatively low in case of the maximum luminance of each pixel, the current value of the voltage value in the minimum gradation and a current difference or a voltage difference between the gradations come to be small if the number of gradations increases.
For example, if the current value of one pixel necessary to emit light with the maximum luminance is 10 μA, a minute current such as 150 pA is controlled to achieve a monochromatic color of 16 bits and 65536 gradations defined by a digital video signal interface standard HDMI (High-Definition Multimedia Interface) 1.3. Accordingly, it is extremely difficult to guarantee accuracy of 150 pA in commercially available cost and size to all of a number of DACs (digital-to-analog converters) arranged in a current driver IC.